Aurelia Butler

Forcing electrons to flow perpendicularly to a heat flow requires an external magnetic field – this is known as the Nernst effect. In a permanently magnetized material (a ferromagnet), an anomalous Nernst effect (ANE) exists that can generate electricity from heat even without a magnetic field. The anomalous Nernst effect scales with the magnetic moment of the ferromagnet. An antiferromagnet, with two compensating magnetic sublattices shows no external magnetic moment and no measurable external magnetic field and therefore should not exhibit any ANE. However, we have recently understood that by the new concept of topology can be applied to achieve large Nernst effects in magnets. In particular, we have learned that the quantity known as the Berry phase is related to the ANE and can greatly increase it. However, the ANE in antiferromagnets is still largely unexplored, in part because the ANE was not thought to exist. Remarkably, a joint research team from the Max Planck Institute for Chemical Physics of Solids in Dresden, Germany, together with collaborators at the Ohio State University and the University of Cincinnati, has found a large anomalous Nernst effect, larger than is known in almost all ferromagnets in YbMnBi2, an antiferromagnet.

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Researchers from Basel and Bochum have succeeded in addressing an apparently unattainable energy transition in an artificial atom using laser light. Making use of the so-called radiative Auger process, they were the first team to specifically excite it. In this process, an electron falls from a higher to a lower energy level and, as a result, emits its energy partly in the form of light and partly by transferring it to another electron. The artificial atoms are narrowly defined areas in semiconductors that could one day form the basis for quantum communication. The findings are described by the team from the University of Basel and Ruhr-Universität Bochum together with colleagues from Münster and Wroclaw in “Nature Communications,” published online on 12 November 2021.
Electrons move between energy states
Atoms consist of a nucleus and electrons that travel around the nucleus. These electrons can assume different energy levels. Electrons that are more tightly bound to the nucleus, i.e. closer to it, have a lower energy than electrons that are further away from the nucleus. However, the electrons can’t assume any arbitrary energy levels — only certain levels are possible.
If an electron acquires energy, for example by absorbing a light particle, i.e. photon, it can be raised to a higher energy level. If an electron falls to a lower energy level, energy is released. This energy can be emitted in the form of a photon. But it can also be transferred to one of the other electrons; in this case, only some of the energy is released as light, the rest is absorbed by the other electron. This process is known as the radiative Auger process.
Exciting a unique energy transition with two lasers
By irradiating light particles, electrons can not only be lifted to a higher energy level; they can also be stimulated to give off energy by an incident light particle. The energy of the incident light particle must always correspond exactly to the difference in the two energy levels between which the electron is to be transferred. The researchers have used two lasers: one moved electrons between a low and a high energy level; the other between the high and a medium energy level. This middle energy level corresponds to a non-equilibrium level: the transfer to the middle level doesn’t exist without a radiative Auger process. In addition, a transition between the low and the medium energy level shouldn’t have occurred, because the relevant light was not irradiated. However, precisely this seemingly impossible transition occurred in reality due to the energy transfer from one electron to another in the radiative Auger process.
The ultrapure semiconductor samples for the experiment were produced by Dr. Julian Ritzmann at Ruhr-Universität Bochum under the supervision of Dr. Arne Ludwig at the Chair for Applied Solid State Physics headed by Professor Andreas Wieck. The measurements were carried out by a team from the University of Basel run by Clemens Spinnler, Liang Zhai, Giang Nguyen and Dr. Matthias Löbl in the group headed by Professor Richard Warburton.
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Materials provided by Ruhr-University Bochum. Note: Content may be edited for style and length. More

Most human diseases can be traced to malfunctioning parts of a cell — a tumor is able to grow because a gene wasn’t accurately translated into a particular protein or a metabolic disease arises because mitochondria aren’t firing properly, for example. But to understand what parts of a cell can go wrong in a disease, scientists first need to have a complete list of parts.
By combining microscopy, biochemistry techniques and artificial intelligence, researchers at University of California San Diego School of Medicine and collaborators have taken what they think may turn out to be a significant leap forward in the understanding of human cells.
The technique, known as Multi-Scale Integrated Cell (MuSIC), is described November 24, 2021 in Nature.
“If you imagine a cell, you probably picture the colorful diagram in your cell biology textbook, with mitochondria, endoplasmic reticulum and nucleus. But is that the whole story? Definitely not,” said Trey Ideker, PhD, professor at UC San Diego School of Medicine and Moores Cancer Center. “Scientists have long realized there’s more that we don’t know than we know, but now we finally have a way to look deeper.”
Ideker led the study with Emma Lundberg, PhD, of KTH Royal Institute of Technology in Stockholm, Sweden and Stanford University.
In the pilot study, MuSIC revealed approximately 70 components contained within a human kidney cell line, half of which had never been seen before. In one example, the researchers spotted a group of proteins forming an unfamiliar structure. Working with UC San Diego colleague Gene Yeo, PhD, they eventually determined the structure to be a new complex of proteins that binds RNA. The complex is likely involved in splicing, an important cellular event that enables the translation of genes to proteins, and helps determine which genes are activated at which times. More

While electrons are well known to carry both charge and spin, only the electric charge portion is utilized as an information carrier in modern electronic devices. However, the limits of modern electronics and the impending end of Moore’s Law have rekindled the interest in the development of “spintronic” devices, which are capable of harnessing the spin of the electrons. It is expected that the widespread adoption of spintronic computing devices can revolutionize information technology similar to the invention of electronics.
One key challenge in spintronics is finding an efficient and sensitive way to electrically detect the electronic spin state. For example, the discovery of giant magnetoresistance (GMR) in the late 1980s, allowed for such functionality. In GMR, a large change in electrical resistance occurs under the magnetic field depending on parallel or antiparallel spin configurations of the ferromagnetic bilayer. The discovery of GMR has led to the development of hard-disk drive technology, which is technically the first-ever mass-produced spintronic device. Since then, discoveries of other related phenomena, including colossal magnetoresistance (CMR) which occurs in the presence of a magnetic field, have advanced our understanding of the interplay between spin and charge degrees of freedom and served as a foundation of emergent spintronic applications.
In the latest issue of the journal Nature, a research team led by Prof. KIM Jun Sung in Center for Artificial Low Dimensional Electron Systems within the Institute for Basic Science (IBS, South Korea) and Physics Department at Pohang University of Science and Technology (POSTECH, South Korea) found a new magnetotransport phenomenon, in the magnetic semiconductor Mn3Si2Te6. The group found that the magnitude of change in resistance can reach as large as a billion-fold under a rotating magnetic field. This unprecedented shift of resistance depending on magnetic field angle is coined as colossal angular magnetoresistance (CAMR). “Unlike the previous magnetotransport phenomena, a huge change in resistance is induced by only rotating the spin direction without altering their configurations. This unusual effect originates from the unique topologically-protected band structure of this magnetic semiconductor,” notes Professor KIM Jun Sung, one of the co-corresponding authors of the study.
Topological materials, a newly discovered class of materials, have become increasingly important in spintronic applications. A topological material refers to a material whose electronic structures are described to be “twisted.” Just as a Mobius strip cannot be unraveled without fundamentally altering its form, the twisted electronic structure in topological materials is preserved unless the system’s symmetry changes. Such topologically protected states can be used to host and control spin information. Along with the recent development of topological materials, topological magnets, where both magnetism and topological electronic states coexist, have been intensively studied. These topological magnets are of great interest with multitudes of potential applications, since their electronic structures are topologically protected but changeable by modulating spin configurations or orientation. This new class of materials offers novel opportunities to couple spin and charge degrees of freedom, which are useful for spin-electronic applications.
In 2018, the research team has reported the discovery of a ferromagnetic semimetal Fe3GeTe2 in Nature Materials. This material was found to have unique nodal-line-shaped band crossing points, and thus classified as a topological magnet. One unique property of this topological magnet is that degeneracy can be lifted in the nodal-line states depending on spin orientation. Extending the idea, the research team has focused on magnetic semiconductors, which possess topological nodal-line states in conduction or valence bands. Again, the band degeneracy of the nodal-line state is sensitive to spin orientation, but in magnetic semiconductors, the lifting of band degeneracy, controlled by spin rotation, can turn the system into either a semiconductor or a metal. Thus charge current flow can be switched on or off by spin rotation, as it is done in conventional semiconductors by applying an electric field.
Identifying the candidate material possessing both ferro- or ferrimagnetism and a topological band degeneracy was the first obstacle. Dr. KIM Kyoo at the Korea Atomic Energy Research Institute (KAERI), used first-principle calculation methods to predict a nodal-line-type band degeneracy in a ferrimagnet Mn3Si2Te6. When he rotated the net magnetic moment of Mn3Si2Te6 in his calculations, the nodal-line degeneracy was lifted, as found in Fe3GeTe2, which is strong enough to induce the bandgap closure. HA Hyunsoo and Prof. YANG Bohm-Jung at the IBS and Seoul National University used the symmetry analysis and found that nodal-line degeneracy of Mn3Si2Te6 is protected by a certain crystalline symmetry, reflecting its topological nature. The constructed Hamiltonian, taking into account both nodal-line states and strong spin-orbit coupling, can capture the calculated changes in the nodal-line states, depending on the spin direction.
Dr. SEO Junho and Dr. De Chandan in Prof. KIM Jun Sung’s research team at the IBS and POTSECH successfully synthesized single crystals of Mn3Si2Te6 and measured their resistance at low temperatures while rotating its spin moments using external magnetic fields. They found that large resistance, reaching gigaohm, turns to tens of milliohm as the magnetic field rotates. This huge change in resistance depending on magnetic field angle has never been observed and is, at least, 100 thousand times larger than previously known magnetic materials that show angular magnetoresistance. LEE Ji Eun and Prof. KIM Jae Hoon in the Department of Physics at Yonsei University in Seoul, South Korea used terahertz absorption measurements to experimentally confirm that the observed huge change in resistance is indeed due to electronic gap closure and the resulting insulator-to-metal transition, as it was theoretically predicted. These theoretical and experimental findings from the close collaboration of the research teams involved proved that the colossal angular magnetoresistance is a direct consequence of spin-polarized nodal-line states and their unique spin-charge coupling.
The newly discovered colossal angular magnetoresistance is expected to be utilized in vector magnetic sensing with high angular sensitivity or efficient electrical readout of the spin state. Furthermore, by exploiting the semiconducting nature of Mn3Si2Te6, a new type of spintronic device can be realized, in which both charge and spin degrees of freedom are modulated by using electric or magnetic fields simultaneously. One of the remaining challenges is how to extend the working temperature range of the colossal angular magnetoresistance up to room temperature. The colossal angular magnetoresistance is considered to be a common property of magnetic topological semiconductors that have a triangular lattice as a structural motif. “In nature, there is a vast possibility of candidate magnetic semiconductors, showing similar or even stronger properties at high temperatures, awaiting theoretical investigation and experimental verification,” noted Professor Yang, one of the co-corresponding authors of the study. More

The ability to precisely control and change properties of a photon, including polarization, position in space, and arrival time, gave rise to a wide range of communication technologies we use today, including the Internet. The next generation of photonic technologies, such as photonic quantum networks and computers, will require even more control over the properties of a photon.
One of the hardest properties to change is a photon’s color, otherwise known as its frequency, because changing the frequency of a photon means changing its energy.
Today, most frequency shifters are either too inefficient, losing a lot of light in the conversion process, or they can’t convert light in the gigahertz range, which is where the most important frequencies for communications, computing, and other applications are found.
Now, researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed highly efficient, on-chip frequency shifters that can convert light in the gigahertz frequency range. The frequency shifters are easily controlled, using continuous and single-tone microwaves.
“Our frequency shifters could become a fundamental building block for high-speed, large-scale classical communication systems as well as emerging photonic quantum computers,” said Marko Lončar, the Tiantsai Lin Professor of Electrical Engineering and senior author of the paper.
The paper outlines two types of on-chip frequency shifter — one that can covert one color to another, using a shift of a few dozen gigahertz, and another that can cascade multiple shifts, a shift of more than 100 gigahertz. More

Scientists seek stable, high-energy batteries designed for the electric grid.
Bringing new sources of renewable energy like wind and solar power onto the electric grid will require specially designed large batteries that can charge when the sun is shining and give energy at night. One type of battery is especially promising for this purpose: the flow battery. Flow batteries contain two tanks of electrically active chemicals that exchange charge and can have large volumes that hold a lot of energy.
For researchers working on flow batteries, their chief concern involves finding target molecules that offer the ability to both store a lot of energy and remain stable for long periods of time.
To find the right flow battery molecules, researchers at the U.S. Department of Energy’s (DOE) Argonne National Laboratory have turned to the power of artificial intelligence (AI) to search through a vast chemical space of over a million molecules. Discovering the right molecules requires optimizing between several different characteristics. “In these batteries, we know that a majority of the molecules that we need will have to satisfy multiple properties,” said Argonne chemist Rajeev Assary. “By optimizing several properties simultaneously, we have a better shot of finding the best possible chemistry for our battery.”
In a new study that follows on from work done last year, Assary and his colleagues in Argonne’s Joint Center for Energy Storage Research modeled anolyte redoxmers, or electrically active molecules in a flow battery. For each redoxmer, the researchers identified three properties that they wanted to optimize. The first two, reduction potential and solvation free energy, relate to how much energy the molecule can store. The third, fluorescence, serves as a kind of self-reporting marker that indicates the overall health of the battery.
Because it is extraordinarily time consuming to calculate the properties of interest for all potential candidates, the researchers turned to a machine learning and AI technique called active learning, in which a model can actually train itself to identify increasingly plausible targets. “We’re essentially looking for needles in haystacks,” said Argonne postdoctoral researcher Hieu Doan. “When our model finds something that looks like a needle, it teaches itself how to find more.”
For the most efficient use of active learning, the researchers started with a fairly small “haystack” — a dataset of 1400 redoxmer candidates whose properties they already knew from quantum mechanical simulations. By using this dataset as practice, they were able to see that the algorithm correctly identified the molecules with the best properties. More